Non-invasive, semi-invasive and invasive ultrasound imaging has been widely used to view tissue structures within a human body, such as the heart structures, the abdominal organs, the fetus, and the vascular system. The semi-invasive systems include transesophageal imaging systems, and the invasive systems include intravascular imaging systems. Depending on the type and location of the tissue, different systems provide better access to or improved field of view of internal biological tissue.
In general, ultrasound imaging systems include a transducer array connected to a multiple channel transmit and receive beamformer. The transmit beamformer applies electrical pulses to the individual transducers in a predetermined timing sequence to generate transmit beams that propagate in predetermined directions from the array. As the transmit beams pass through the body, portions of the acoustic energy are reflected back to the transducer array from tissue structures having different acoustic characteristics. The receive transducers (which may be the transmit transducers operating in a receive mode) convert the reflected pressure pulses into corresponding electrical RF signals that are provided to the receive beamformer. Due to different distances from a reflecting point to the individual transducers, the reflected sound waves arrive at the individual transducers at different times, and thus the RF signals have different phases.
The receive beamformer has a plurality of processing channels with compensating delay elements connected to a summer. The receive beamformer selects the delay value for each channel to combine echoes reflected from a selected focal point. Consequently, when delayed signals are summed, a strong signal is produced from signals corresponding to this point. However, signals arriving from different points, corresponding to different times, have random phase relationships and thus destructively interfere. The receive beamformer selects such relative delays that control the orientation of the receive beam with respect to the transducer array. Thus, the receive beamformer can dynamically steer the receive beams to have desired orientations and can focus them at desired depths. The ultrasound system thereby acquires acoustic data.
To view tissue structures in real-time, various ultrasound systems have been used to generate two-dimensional or three-dimensional images. A typical ultrasound imaging system acquires a two-dimensional image plane that is perpendicular to the face of the transducer array applied to a patient's body. To create a three-dimensional image, the ultrasound system must acquire acoustic data over a three-dimensional volume by, for example, moving a one-dimensional (or a one-and-half dimensional) transducer array over several locations. Alternatively, a two-dimensional transducer array can acquire scan data over a multiplicity of image planes. In each case, the system stores the image plane data for reconstruction of three-dimensional images. However, to image a moving organ, such as the heart, it is important to acquire the data quickly and to generate the images as fast as possible. This requires a high frame rate (i.e., the number of images generated per unit time) and fast processing of the image data. However, spatial scanning (for example, when moving a one-dimensional array over several locations) is not instantaneous. Thus, the time dimension is intertwined with the three space dimensions when imaging a moving organ.
Several ultrasound systems have been used to generate 3D images by data acquisition, volume reconstruction, and image visualization. A typical ultrasound system acquires data by scanning a patient's target anatomy with a transducer probe and by receiving multiple frames of data. The system derives position and orientation indicators for each frame relative to a prior frame, a reference frame or a reference position. Then, the system uses the frame data and corresponding indicators for each frame as inputs for the volume reconstruction and image visualization processes. The 3D ultrasound system performs volume reconstruction by defining a reference coordinate system within which each image frame in a sequence of the registered image frames. The reference coordinate system is the coordinate system for a 3D volume encompassing all image planes to be used in generating a 3D image. The first image frame is used to define the reference coordinate system (and thus the 3D volume), uses either three spherical axes (rv, Θv, and φv axes) or three orthogonal axes (i.e., xv, yv and zv axes). Each image frame is a 2D slice (i.e., a planar image) has two polar axes (i.e., ri and Θi axes) or two orthogonal axes (i.e., xi and yi), where i is the i-th image frame. Thus, each sample point within an image plane has image plane coordinates in the image plane coordinate system for such image plane. To register the samples in the reference coordinate system, the sample point coordinates in the appropriate image plane coordinate system are transposed to the reference coordinate system. If an image plane sample does not occur at specific integer coordinates of the reference coordinate system, the system performs interpolation to distribute the image plane sample among the nearest reference coordinate system points.
To store sample data or the interpolated values derived from the sample data, the system allocates memory address space, wherein the memory can be mapped to the reference coordinate system. Thus, values for a given row of a given reference volume slice (taken along, for example, the z-axis) can be stored in sequential address locations. Also, values for adjacent rows in such slice can be stored in adjacent first memory address space. The system performs incremental reconstruction by computing a transformation matrix that embodies six offsets. There are three offsets for computing the x, y, and z coordinates in the x-direction (along the row of the image), and three offsets for computing the x, y, and z coordinates in the y-direction (down the column of the image). Then, the system computes the corners of the reconstruction volume and compares them with the coordinates of the bounding volume. Next, the system determines the intersecting portion of the acquired image and the bounding coordinates and converts them back to the image's coordinate system. This may be done using several digital signal processors.
Furthermore, the system can compute an orthogonal projection of the current state of the reconstruction volume. An orthogonal projection uses simpler computation for rendering (no interpolations need to be computed to transform from the reference coordinate system to a displayed image raster coordinate system). The system can use a maximum intensity projection (MIP) rendering scheme in which a ray is cast along the depth of the volume, and the maximum value encountered is the value that is projected for that ray (e.g., the value used to derive a pixel for a given raster point on the 2D image projection). The system incrementally reconstructs and displays a target volume in real time. The operator can view the target volume and scan effectiveness in real time and improve the displayed images by deliberately scanning desired areas repeatedly. The operator also can recommence volume reconstruction at the new viewing angle.
The image visualization process derives 2D image projections of the 3D volume over time to generate a rotating image or an image at a new viewing angle. The system uses a shear warp factorization process to derive the new 2D projection for a given one or more video frames of the image. For each change in viewing angle, the process factorizes the necessary viewing transformation matrix into a 3D shear which is parallel to slices of the volume data. A projection of the shear forms a 2D intermediate image. A 2D warp can be implemented to produce the final image, (i.e., a 2D projection of the 3D volume at a desired viewing angle). The system uses a sequence of final images at differing viewing angles to create a real-time rotating view of the target volume.
Other systems have been known to utilize power Doppler images alone in a three dimensional display to eliminate the substantial clutter caused by structural information signals. Such Doppler system stores Doppler power display values, with their spatial coordinates, in a sequence of planar images in an image sequence memory. A user can provide processing parameters that include the range of viewing angles. For instance, the user can input a range of viewing angles referenced to a line of view in a plane that is normal to the plane of the first image in the sequence, and a range increment. From these inputs the required number of three dimensional projections is computed. Then, this system forms the necessary sequence of maximum intensity projections by first recalling the planar Doppler power images from the image sequence memory for sequential processing by a scan converter and display processor. The processor rotates each planar image to one of the viewing angles projected back to the viewing plane.
The Doppler system accumulates the pixels of the projected planar images on a maximum intensity basis. Each projected planar image is overlaid over the previously accumulated projected images but in a transposed location in the image plane which is a function of the viewing angle and the interplane spacing: the greater the viewing angle, the greater the transposition displacement from one image to the next. The display pixels chosen from the accumulated images are the maximum intensity pixels taken at each point in the image planes from all of the overlaid pixels accumulated at each point in the image. This effectively presents the maximum intensity of Doppler power seen by the viewer along every viewing line between the viewer and the three dimensional representation.
This system can rotate, project, transpose, overlay, and choose the maximum intensities at each pixel for all of the planar images, and then store in the image sequence memory the resulting three dimensional representation for the viewing angle. The stored three dimensional sequence is available for recall and display upon command of the user. As the sequence is recalled and displayed in real time, the user can see a three dimensional presentation of the motion or fluid flow occurring in the volumetric region over which the planar images were acquired. The volumetric region is viewed three dimensionally as if the user were moving around the region and viewing the motion or flow from changing viewing angles. The viewer can sweep back and forth through the sequence, giving the impression of moving around the volumetric region in two directions.
It has also been known to utilize a modified two dimensional ultrasonic imaging system to provide three dimensional ultrasonic images. Such three dimensional ultrasonic imaging system can use conventional two dimensional ultrasonic imaging hardware and a scan converter. The two dimensional ultrasonic imaging system acquires a plurality of two dimensional images. This system processes the images through scan conversion to approximate their rotation to various image planes and projection back to a reference plane, which can be the original image plane. Conventional scan conversion hardware can be used to rescale the sector angle or depth of sector images, or the aspect ratio of rectangular images. This system projects a plurality of planes for each image and then stored them in a sequence of combined images, wherein each combined image comprises a set of corresponding projected images offset with respect to each other. Each combined image is a different view of a three dimensional region occupied by the planar image information.
The above system can replay the sequence of combined images on a display to depict the three dimensional region as if it is rotating in front of a viewer. Furthermore, the system can recall the stored combined images on the basis of the three dimensional viewing perspectives and displayed sequentially in a three dimensional presentation.
There are several medical procedures where ultrasound imaging systems are not yet widely used. Currently, for example, interventional cardiologists use mainly fluoroscopic imaging for guidance and placement of devices in the vasculature or in the heart. These procedures are usually performed in a cardiac catheterization laboratory (Cathlab) or an electrophysiology laboratory (Eplab). During cardiac catheterization, a fluoroscope uses X-rays on a real-time frame rate to give the physician a transmission view of a chest region, where the heart resides. A bi-plane fluoroscope, which has two transmitter-receiver pairs mounted at 90° to each other, provides real-time transmission images of the cardiac anatomy. These images assist the physician in positioning various catheters by providing him (or her) with a sense of the three-dimensional geometry of the heart.
While fluoroscopy is a useful technique, it does not provide high quality images with good contrast in soft tissues. Furthermore, the physician and the assisting medical staff need to cover themselves with a lead suit and need to reduce the fluoroscopic imaging time whenever possible to lower their exposure to X-rays. In addition, fluoroscopy may not be available for some patients, for example, pregnant women, due to the harmful effects of the X-rays. Recently, transthoracic and transesophageal ultrasound imaging have been very useful in the clinical and surgical environments, but have not been widely used in the Cathlab or Eplab for patients undergoing interventional techniques.
Therefore there is a need for transesophageal or transnasal, transesophageal ultrasound systems and methods that can provide fast and computationally inexpensive real-time imaging. The images should enable effective visualization of the internal anatomy that includes various structures and provide selected views of the tissue of interest. An ultrasound system and method providing anatomically correct and easily understandable, real-time images would find additional applications in medicine.